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Electronic and Optical Properties of Silicon Nanocrystals

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Electronic and Optical Properties of Silicon Nanocrystals Electronic and Optical Properties of Silicon Nanocrystals Ceyhun Bulutay1 and Stefano Ossicini2 1Department of Physics, Bilkent University, Bilkent, Ankara 06800, Turkey 2CNR-INFM-S3 and Dipartimento di Scienze e Metodi dell’Ingegneria, Universit`adi Modena e Reggio Emilia, via Amendola 2 Pad. Morselli, I-42100 Reggio Emilia, Italy. The previous decades repeatedly witnessed claims that silicon would be sidelined by various alternative material systems. In the long run, none of these claims turned out to be the case. Taking into account the changing societal and technological needs, there are strong indications that the nanocrystalline form of silicon will provide a critical support to sustain this domination as the all-purpose material of choice. The purpose of this study is to offer a comprehensive theoretical overview of the electronic structure and optical properties of Si nanocrystals (Si NCs). Starting with small NCs, we use a first-principles methodology taking into account the structural relaxations. We go beyond the one-particle approach including the self-energy corrections, by means of the GW approximation, and the excitonic effects, through the solution of the Bethe-Salpeter equation. This new approach, where many-body effects are combined with a study of the structural distortion due to the impurity atoms in the excited state, allows to calculate accurately the Stokes shift between absorption and photoluminescence spectra. Regarding the interface and impurity effects, first, the hydrogenated Si NCs are considered, followed by the discussion of oxidation, and of (co-)doping and finally, of the crystalline as well as amorphous embedding matrix. For the larger Si NCs embedded in a wide band-gap matrix containing of the order of 10,000’s of atoms, an atomistic semi-empirical pseudopotential approach is utilized. A validation of the semi-empirical results is given by comparing with the experimental and first-principles data on the effective optical gap and the radiative lifetime. The linear optical absorption properties are discussed for interband, intraband and excited-state configurations. Next, third-order nonlinear optical susceptibilites are computed with full wavelength dependence and their size-scaling trends are identified. Finally, the quantum- confined Stark effect in Si NCs is analysed which reveals the discrepancy of the Stark shifts of the valence and conduction states. I. INTRODUCTION Several strategies have been researched over the last years for light generation and amplification in silicon. One of the most promising is based on silicon nanocrystals (Si NCs) with the idea of taking advantage of the reduced dimensionality of the nanocrystalline phase (1-5 nm in size) where quantum confinement, band folding and surface effects play a crucial role.1,2 Indeed, it has been found that the Si NC band-gap increases with decreasing size and a photoluminescence external efficiency in excess of 23% has been obtained.3 Si NC based LED with high efficiency have been obtained by using Si NC active layers4 and achieving separate injection of electrons and holes.5 Moreover, optical gain under optical pumping has been already demonstrated in a large variety of experimental conditions.6–11 After the initial impulse given by the pioneering work of Canham on photoluminescence (PL) from porous Si,12 nanostructured silicon has received extensive attention. This activity is mainly centered on the possibility of getting relevant optoelectronic properties from nanocrystalline Si. The huge efforts made towards matter manipulation at the nanometer scale have been motivated by the fact that desirable properties can be generated just by changing the system dimension and shape. Investigation of phenomena such as the Stokes shift (difference between absorption and emission energies), the PL emission energy vs nanocrystals size, the doping properties, the radiative lifetimes, the non-linear optical properties, the quantum-confined Stark effect etc. can give a fundamental contribution to the understanding of how the optical response of such systems can be tuned. An interesting amount of work has been done regarding excited Si NCs,2,13–23 but a clear comprehension of some aspects is still lacking. The question of surface effects, in particular oxidation, has been addressed in the last years. Both theoretical calculations and experimental observations have been applied to investigate the possible active role of the interface on the optoelectronic properties of Si NCs. Different models have been proposed: Baierle et al.24 have considered the role of the surface geometry distortion of small hydrogenated Si clusters in the excited state. Wolkin et al.25 have observed that oxidation introduces defects in the Si NC band-gap which pin the transition energy. They claimed the formation of a Si=O double bond as the pinning state. The same conclusion has been recently reached by other authors,26–29 whereas Vasiliev et al.30 have pointed out that similar results can be obtained also for O connecting two Si atoms (single bond) at the Si NC surface. The optical gain observed in Si NC embedded in SiO2 has given a further impulse to these studies. Interface radiative states have been suggested to play a key role in the mechanism of population inversion at the origin of the gain.6,8,31 Thus the study of the nature and the properties of the interface between the Si NC and the SiO2 host matrix has become crucial. The calculation of the electronic and optical properties of nanostructures is a difficult task. First-principle studies are very demanding and in order to investigate very large systems empirical methods are needed. In this paper we present and resume a comprehensive study of the structural, electronic and optical properties of undoped and doped Si nanostructures terminated by different interfaces and, in particular, embedded in silicon dioxide matrix. For smaller nanocrystals we will present ab initio results, in particular the absorption and emission spectra and the effects induced by the creation of an electron-hole pair are calculated and discussed in detail including many-body effects. The aim is to investigate in a systematic way the structural, electronic and stability properties of silicon nanostructures as a function of size and capping species well as pointing out the main changes induced by the nanostructure excitation. The indisputable superiority of the first-principles approaches is gloomed by their applicability to systems of less than a thousand atoms with the current computer power. On the other hand, fabricated NCs of sizes 2-5 nm embedded in an insulating host matrix require computationally more feasible techniques that can handle more than 10,000 atoms including the surrounding matrix atoms. There exist several viable computational approaches for low-dimensional structures with a modest computational budget. The most common ones are the envelope function k p,32 semi- empirical tight-binding,33 and semi-empirical pseudopotential techniques.34 The decision of which one to· use should be made according to the accuracy demands, but a subjective dimension is also brought by the established biases of the particular practitioners. As the simplest of all, the envelope function k p approach lacks the atomistic touch and more importantly, both qualitative and significant quantitative errors were· identified mainly derived from the states that were not accounted by the multiband k p Hamiltonian.35 As an atomistic alternative, the semi-empirical tight binding approach has been successfully employed· by several groups; see, Ref. 33 and references therein. Its traditional rival has always been the semi-empirical pseudopotential approach. About a decade ago, Wang and Zunger proposed a more powerful technique that solves the pseudopotential-based Hamiltonian using a basis set formed by the linear combination of bulk bands of the constituents of the nanostructure.36,37 Its main virtue is that it enables an insightful choice of a basis set with moderate number of elements. It should be mentioned that, the idea of using bulk Bloch states in confined systems goes back to earlier times; one of its first implementations being the studies of Ninno et al.38,39 Up to now, it has been tested on self-assembled quantum dots,36,37 superlattices,40,41 and high-electron mobility transistors.42 In the context of Si NCs, very recently it has been used for studying the effects of NC aggregation,43 the linear,44 and third-order nonlinear optical properties,45 and also for characterizing the Auger recombination and carrier multiplication.46 The fact that it is a pseudopotential-based method makes it more preferable over the empirical tight binding technique for the study of optical properties. For these reasons, in the case of large NCs, we shall make use of the linear combination of bulk bands technique. For both small NCs analysed with ab initio techniques, and larger NCs dealt with atomistic semi-empirical approaches, a comparison with the experimental outcomes will be presented and discussed, whenever possible. The organization of this review is based on two main sections for discussing the methodology and results of small and large NCs. A description of the theoretical methods used in the ab initio calculations is given in Sec. II. The first- principle study of the physical systems is then presented starting from the analysis of hydrogenated Si NC (Sec. II A). We then consider the effect of oxidation (Sec. II B), of doping (Sec. II C) and finally of an embedding matrix (Sec. II D). As for the larger NCs embedded in a wide band-gap matrix, in Sec. III, first a theoretical framework of the semi- empirical approach is presented. Next, the comparisons of our results for the case of effective optical gap (Sec. III A), and radiative lifetime (Sec. III B) are presented. This is followed by the linear optical absorption properties (Sec. III C), where our interband, intraband and excited-state absorption results are summarized.
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